1. Field of the Invention
This invention relates to a radio wave shielding material and a method of producing the same.
2. Description of the Prior Art
Electron applied instruments and machines (hereinafter referred to as electron application apparatus) such as personal computer, office automation (OA) instrument, factory automation (FA) machine, home automation (HA) instrument, television game machine, personal radiocommunication system and the like are in wide spread use in offices, factories, homes and public facilities. For this reason, radio wave interference against other electron instruments and machines due to radiation leakage from the above electron application apparatus, such as erroneous operation, communication interference and the like have created problems. In U.S.A., the Federal Communication Commission (FCC) has announced regulation values of the maximum acceptable field strength in radio wave leakage from the above electron application apparatuses dividing into class A (commercial use) and class B (home use), which have completely been practised from Oct. 1, 1983. In Japan, a conference on the above problems was made in the radio wave council in Ministry of Posts and Telecommunications and consequently the regulation value of the leaked radio waves will be established within a few years.
Thus, countermeasures for shielding such radio waves are urgently needed in the field of manufacturing the electron application apparatus, and it is a requirement to develop radio wave shielding materials having excellent shielding performance.
In order to shield the leakage of radio wave from the electron application apparatus to the outside, a metal or a composite material having an electric conductivity is used as a material constituting the casing of the electron application apparatus and is sufficiently grounded, whereby this object can be achieved. In this case, however, it is important to prevent the leakage of radio wave, particularly high frequency pulsive wave from the joint portion of the casing or gaps at contact portions with an openable door and the like. Therefore, a hollow extrusion molded article of a conductive rubber or a gasket or a packing composed of a knitted body of metal wires has hitherto been used in the gap of the conductive casing for shielding the radio wave leaked from such gap, which has various drawbacks as will be mentioned.
That is, the hollow extrusion molded articles made from rubber materials kneaded with a conductive filler, typically conductive silicone rubber as a radio wave shielding gasket have a volume resistivity of about 100 .OMEGA..multidot.cm as a practical limit and are low in the conductivity at a high frequency range, so that they are insufficient in radio wave shielding performance. On the other hand, since the gasket composed of the knitted body of metal wires is high in rigidity and bad in the adhesion, a gap is formed between the casing and the gasket, from which radio waves leaks. Further, the conduction mechanism of the knitted body is based on the contact between the wires, so that the electric resistance at a high frequency range becomes very high and the radio wave shielding property lowers.
Particularly, the radio wave shielding materials suitable for use in the computer, measuring instrument, audio apparatus and the like are strongly required to have a high radio wave shielding property (60.about.80 dB) over a wide frequency range of from low frequency (30 MHz) to high frequency (500.about.1,000 MHz) and a good compression modulus capable of flexibly fitting into or closably contacting with the gap of the casing for the electron application apparatus.